In recent years, an RFID (Radio Frequency Identification) system has been widely used for various purposes. The RFID system includes a wireless tag and a reader, and performs various functions via wireless communications between the tag and the reader.
The wireless tag for use in the RFID system is categorized into a passive tag that contains no battery and an active tag that contains a battery. The passive tag is used as a wireless tag for wireless communications between itself and a reader that is close to it (e.g., for use as a pre-paid card). On the other hand, the active tag is used as a wireless tag for wireless communications between itself and a reader that is not close to it (e.g., for use as a tag carried by a user in a presence management system or as a tag attached to a commercial product in an inventory management system). The presence management system is disclosed in, for example, Patent Literature 1.
It is necessary that the wireless tag include an antenna for wireless communications between itself and a reader. As the antenna included in the wireless tag, a small loop antenna is often used regardless of whether the wireless tag is an active tag or a passive tag. However, the active tag including a small loop antenna has too small a radiative power that it may cause a problem in wireless communications between the tag and a reader that is not close to the tag.
One way to solve such a problem is to use a small dipole antenna or a small monopole antenna instead of the small loop antenna. It should be noted here that the small dipole antenna and the small monopole antenna mean a dipole antenna and a monopole antenna, respectively, each having an antenna element whose total length ρ is much smaller than its resonant wavelength λ (i.e., ρ<<λ). The radiative power of the small loop antenna is proportional to (ρ/λ)4, whereas the radiative power of each of the small dipole and monopole antennas is proportional to (ρ/λ)2. That is, the radiative power of each of the small dipole and monopole antennas is greater than the radiative power of the small loop antenna. However, both of these small antennas, which satisfy ρ<<λ, can only achieve a limited level of radiative power.
On the other hand, a half-wave dipole antenna satisfying ρ=λ/2 and a quarter-wave monopole antenna satisfying ρ=λ/4 are known to have better radiation efficiencies than the above-mentioned small antennas. An active tag including a half-wave dipole antenna is, for example, one that is disclosed in Patent Literature 2. The active tag disclosed in Patent Literature 2 includes a planar half-wave dipole antenna and thereby achieves a sufficient radiative power and also achieves a small thickness.